The inkjet recording system has an advantage over an electrophotographic system in terms that the price of main body is held down and that printing cost can be held down because of low power consumption. Thus, the inkjet recording system is widespread for personal use and office use. An aqueous ink is a mainstream ink in terms of safety and environment-friendliness, and the ink containing a coloring material, water, and a water-soluble solvent is mainly used. However, an image printed by the inkjet recording system using such ink tends to bleed on plain paper, compared to an image printed by the electrophotographic system. Therefore, image enhancement has been a problem for the purpose of achieving a superior image quality.
When recording is performed by scanning with an ink ejection head in the inkjet recording system, the inkjet recording system is inferior in the recording speed to the electrophotographic system. Thus, in order to increase the recording speed, various methods are implemented; for example, the number of ejection nozzles are increased, or recording is performed in both an outward direction and an inward direction of scanning (bidirectional printing) so as to reduce the number of scanning.
However, image density may be seriously decreased due to the order of superimposing the ink colors. For example, the case where at first a black ink droplet is attached onto a medium and then a color ink droplet is attached thereon (for example, in an outward direction), and the case where at first a color ink droplet is attached onto a medium and then a black ink droplet is attached thereon (for example, in an inward direction) are compared, density is seriously decreased in the latter. As shown in FIG. 1B, it is considered that this phenomenon occurs because an ink droplet attached onto paper at first (α) remains near a paper surface, and an ink droplet attached thereon later (β) penetrates into the paper.
This phenomenon causes difference in image density between the outward direction and the inward direction in bidirectional printing. As an effective means for preventing occurrence of difference in image density between the outward direction and the inward direction due to the change of the ejection order in the case where a black image is formed by mixing a color ink droplet and a black ink droplet, when firstly the color ink droplet is attached onto a medium, the black ink droplet is formed into aggregation on a surface of the medium so as to prevent the black ink droplet attached thereon later from penetrating into the medium.
An invention of Patent Literature 1 discloses that when a black image is formed, a color ink is ejected on a printing portion of black ink, namely, any one of cationic inks of cyan, magenta and yellow is superimposed on an anionic black ink, so that a coloring material of the black ink is insolubilized, thereby increasing image density. However, this method limits nozzle arrangement depending on the color of the cationic ink.
An invention of Patent Literature 2 discloses a method of forming a black image by superimposing an anionic black ink and a cationic light color ink. However, in this method, another head for the light color ink needs to be arranged. Thus, an apparatus is enlarged, and the number of cartridge is increased, raising cost.
In this way, it has been proposed that a method of aggregating the black ink by taking advantage of difference in ionicity. However, an inkset, which does not cause difference in image density depending on the order of ink ejection, and is excellent in ejection stability and image quality, has not been achieved at present.
On the other hand, Patent Literature 3 discloses an inkset consisting of a black ink and color inks of cyan, magenta and yellow, which including the same surfactant as that used in the present invention. However, decrease of color difference in bidirectional printing, which is the problem of the present invention, and means for solving the problem is not disclosed nor suggested. This is also apparent from claim 5, in which the same types of surfactants are used in the black ink and the color inks, which is not the case of the present invention.